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several millions of tons per square inch and the period is a fraction of a thousandth part of a second. Hence the shattering effect of these High Explosives.

Such violence of explosive force is just what is needed for shells, provided that the explosives are not too sensitive, or in other words that they can stand the shock of the discharge of the gun without exploding. But here a strange peculiarity shews itself. They have two ways of exploding. The one gives a comparatively mild explosion comparable say with that of gunpowder, the other a fierce detonation the violence of which is akin to that of guncotton. No one has penetrated the mystery of this. It undoubtedly depends on the initial disturbance which sets the explosive off. If that is of a sufficiently intense type and is rightly communicated to the mass of the explosive it produces detonation, and the shell is rent to pieces. If not we have only an explosion of ordinary violence which opens out the shell but does little more. It needed much research to give us practical control of high explosives in this respect. In the